1. Field of the Invention
The present invention relates to a motor driven by a direct-current power supply, and more particularly to an inner-rotor type direct current motor, wherein the rotor is arranged inside the stator having a plurality of field magnets which generate a magnetic field.
2. Description of the Related Art
A direct current motor with brushes is used as, for example, a motor section of an motor-blower for use in a battery-driven electric vacuum cleaner. The direct current motor has a stator including a stator core, which defines a circular rotor through hole, and two field magnets attached on the circular inner periphery of the stator core.
The field magnets are both arc-shaped. The field magnets may be of the type arc-shaped in advance and attached by adhesive to the inner periphery of the stator core or the type injection-molded to an arc shape and attached to the inner periphery of the stator core. The field magnet of the latter type is made of magnetic particle-mixed synthetic resin, and commonly called a plastic magnet.
A part of the circular inner periphery of the stator core to which the field magnets are attached is exposed through a gap between the end portions of the field magnets (hereinafter referred to as magnet end portions). The field magnets are magnetized in the thickness directions. Here the thickness direction means radial directions connecting the inner periphery surface and the outer periphery surface of the stator core.
At the magnet end portions of the magnetized field magnets, magnetic flux flows from the north pole to the south pole. The flow of the magnetic flux is called magnetic flux leakage. The magnetic flux leakage is one of the factors that reduce the torque of the rotor arranged inside the stator. Therefore, it is desirable to reduce the magnetic flux leakage as little as possible.
As described before, the inner periphery of the stator core is circular. Hence, the portion of the inner periphery of the stator core that exposed through the gap between the magnet end portions of the adjacent field magnets is located on an extension of the rear surfaces of the field magnets. The distance from the portion of the inner periphery between the magnet end portions to the surface of each magnet end is short. Particularly in the case where the field magnet is formed of a rare-earth magnet having a high energy product, the magnet is thin. For this reason, if a rare-earth magnet is used as the field magnet, the distance is shorter.
The stator core easily passes magnetic flux by nature. As described above, the exposed portion of the inner periphery of the core is located at a very short distance from the surfaces of the magnet ends. In this structure, the magnetic flux from the north pole to the south pole at the magnet end portions easily passes through the exposed portion of the inner periphery. Thus, when there is much magnetic flux leakage at the magnet end portions, the torque that rotates the rotor is reduced.
To injection-mold a plastic magnet and attach it to the inner periphery of the stator core, a pair of molding dies are used. The molding dies are used such that they sandwich the stator core in the thickness direction. One of the molding dies has an insertion die portion. The stator core has a rotor through hole in a central portion thereof. In the injection molding, the insertion die portion is inserted into the rotor through hole. The inserted insertion die portion forms a cavity between itself and the inner periphery of the stator core.
To form the cavity, the insertion die portion is brought into contact with the inner periphery of the stator core. The cavity is formed so as to correspond to the shape of the field magnet. The cavity is filled with the plastic magnet by injection. As a result, the field magnet is injection molded to the inner periphery of the stator core.
The insertion die portion, which is inserted in and removed from the rotor through hole in the injection molding, is brought into contact with the inner periphery of the stator core. Thus, since the insertion die portion inevitably wears with time, the lifetime of the molding die is short.
As the wear of the insertion die portion proceeds, thin burr-like portions projecting from the ends of the field magnet are formed. The projecting portions easily peel after molding. When the projecting portions begin to exfoliate, the field magnet may be liable to peel off from the inner periphery of the stator core, triggered by the exfoliation.
An object of the present invention is to provide a direct current motor, which suppresses the magnetic flux leakage at the magnet end portions of the field magnets and increases the torque of the rotor.
In a direct current motor of the present invention, a plurality of field magnets are attached to the inner periphery of a stator core, which defines a rotor through hole. The stator core has recesses facing magnetic end portions of the field magnets. The recesses open to the rotor through hole. Each recess has a surface (herein after referred to as a recess surface), which is away from the surface of the magnetic end portions, and set back toward the outer periphery of the stator core from the rear surface of the magnet end portion.
In this invention, the stator core may be formed of a magnetic plate of magnetic metal, for example, a silicon steel plate. The field magnet may be formed not by injection molding, or by injection molding plastic magnet. It is preferable that the field magnet be formed of a rare-earth magnet, since the torque can be more increased or the direct current motor can be more compact.
In this invention, the recess may be extended to the rear sides of the magnet end portions. The recess preferably extends across the magnet end portions of the adjacent field magnets. However, recesses may be formed to correspond to the respective magnet end portions.
According to the direct current motor of the present invention, since the recesses are formed in the stator core, the distance between the recess surface and the surface of a magnet end portion is lengthened. Magnetic resistance is obtained by air in the recess. Therefore, the magnetic flux flowing from the north pole to the south pole at the magnet end portions through the stator core, which easily passes magnetism, can be reduced. Accordingly, the magnetic flux can be efficiently used, thereby increasing the torque of the rotor.